Mixed perovskites have achieved substantial successes in boosting solar cell efficiency, but the complicated perovskite crystal formation pathway remains mysterious. Here, the detailed crystallization process of mixed perovskites (FA0.83MA0.17Pb(I0.83Br0.17)3) during spin‐coating is revealed by in situ grazing‐incidence wide‐angle X‐ray scattering measurements, and three phase‐formation stages are identified: I) precursor solution; II) hexagonal δ‐phase (2H); and III) complex phases including hexagonal polytypes (4H, 6H), MAI–PbI2–DMSO intermediate phases, and perovskite α‐phase. The correlated device performance and ex situ characterizations suggest the existence of an “annealing window” covering the duration of stage II. The spin‐coated film should be annealed within the annealing window to avoid the formation of hexagonal polytypes during the perovskite crystallization process, thus achieving a good device performance. Remarkably, the crystallization pathway can be manipulated by incorporating Cs+ ions in mixed perovskites. Combined with density functional theory calculations, the perovskite system with sufficient Cs+ will bypass the formation of secondary phases in stage III by promoting the formation of α‐phase both kinetically and thermodynamically, thereby significantly extending the annealing window. This study provides underlying reasons of the time sensitivity of fabricating mixed‐perovskite devices and insightful guidelines for manipulating the perovskite crystallization pathways toward higher performance.

Dopants and defects are important in semiconductor and magnetic devices. Strategies for controlling doping and defects have been the focus of semiconductor physics research during the past decades and remain critical even today. Co-doping is a promising strategy that can be used for effectively tuning the dopant populations, electronic properties, and magnetic properties. It can enhance the solubility of dopants and improve the stability of desired defects. During the past 20 years, significant experimental and theoretical efforts have been devoted to studying the characteristics of co-doping. In this article, we first review the historical development of co-doping. Then, we review a variety of research performed on co-doping, based on the compensating nature of co-dopants. Finally, we review the effects of contamination and surfactants that can explain the general mechanisms of co-doping.

Based on the density functional theory with hybrid functional approach, we have studied the structural and thermodynamic stabilities of Cu2M SnX4 (M = Zn, Mg, and Ca; X = S and Se) alloy, and have further investigated the electronic and optical properties of stable Cu2MgSnS4 and Cu2MgSnSe4 phases. Thermal stability analysis indicates that Cu2MgSnS4 and Cu2MgSnSe4 are thermodynamically stable, while Cu2CaSnS4 and Cu2CaSnSe4 are unstable. The ground state configuration of the compound changes from kesterite into stannite structure when Zn atoms are substitued by larger Mg or Ca atoms. An energy separation between stannite and kesterite phase similar to that of CZTS is observed. Calculated electronic structures and optical properties suggest that Cu2MgSnS4 and Cu2MgSnSe4 can be efficient photovoltaic materials. arXiv:1509.06230v2 [cond-mat.mtrl-sci] 3 Dec 2015

The accurate absolute surface energies of (0001)/(0001 ̅ ) surfaces of wurtzite structures are crucial in determining the thin film growth mode of important energy materials. However, the surface energies still remain to be solved due to the intrinsic difficulty of calculating dangling bond energy of asymmetrically bonded surface atoms. In this study, we used a pseudo-hydrogen passivation method to estimate the dangling bond energy and calculate the polar surfaces of ZnO and GaN. The calculations were based on the pseudo chemical potentials obtained from a set of tetrahedral clusters or simple pseudo-molecules, using density functional theory approaches. And the surface energies of (0001)/(0001 ̅ ) surfaces of wurtzite ZnO and GaN we obtained showed relatively high self-consistencies. A wedge structure calculation with a new bottom surface passivation scheme of group I and group VII elements was also proposed and performed to show converged absolute surface energy of wurtzite ZnO polar surfaces, and the result were also compared with the above method. These calculations and comparisons may provide important insights to crystal growths of the above materials, thereby leading to significant performance enhancements of semiconductor devices.

Determining accurate absolute surface energies for polar surfaces of semiconductors has been a great challenge in decades. Here, we propose pseudo-hydrogen passivation to calculate them, using density functional theory approaches. By calculating the energy contribution from pseudo-hydrogen using either a pseudo molecule method or a tetrahedral cluster method, we obtained (111)/(111) surfaces energies of Si, GaP, GaAs, and ZnS with high self-consistency. This method quantitatively confirms that surface energy is determined by the number and the energy of dangling bonds of surface atoms. Our findings may greatly enhance the basic understandings of different surfaces and lead to novel strategies in the crystal growth.Absolute surface energies are fundamental physical quantities of solid surfaces with broad implications [1][2][3][4][5] . Equilibrium shapes and morphologies 4,5 , equilibrium growth rates 6-8 , as well as device performance of semiconductors 9 , are directly related to them. The wetting conditions of thin films or super lattices are also determined by these quantities 10 . Novel controlling strategies of growth modes (e.g. by strain or surfactants) are also often determined by them [10][11][12][13][14][15][16][17][18] . Therefore, determining accurate absolute surface energy is essential in understanding growth processes and in improving the performance of thin-film devices 3,19 .Absolute surface energies of symmetric non-polar surfaces can be calculated with a standard slab method 3 . However, for asymmetric polar surfaces such as zinc blende (111) and ( ) 111 surfaces, it is extremely difficult to separate the anion and cation terminated surfaces, due to the asymmetric nature of slabs 3,19 . Although surface reconstructions and surface kinetic processes can be calculated by passivating the conjugate surfaces with pseudo-H atoms, the absolute surface energy can't be obtained unless the energy of the passivated surface is known 3,19 . Despite the standard treatment of pseudo H passivation in surface calculations, a detailed analysis of the bonding between the surface atom and the pseudo H is lacking and may serve as a key to solve the difficult absolute surface energy problem. Chetty and Martin 21 first proposed local energy density approach to determine the absolute surface energies of polar surfaces. However, this approach suffers from a non-trivial approximation of the local energy density 3 , which leads to a large disagreement among calculations using exactly the same method 20 . Another early approach to calculate the absolute surface energies of zinc blende (111) surfaces is to construct a wedge structure and then calculate one polar surface without involving its conjugate surfaces 3 . Based on surface energies calculated by this approach, a common dangling bond rule was also proposed, which states that energies of surface atoms with similar electronic environment are the same regardless of the different surface orientations 3 . Following this method, absolute surface energies of quite a few ...